1. Field of the Invention
The invention relates to an electronic switching device, which operates preferably without contact, and includes a sensor element, a clock generator, which can be provided internally or which can be connected externally, an evaluation circuit, and an electronic or electromechanical switch, which is provided on an output side.
2. Description of Related Art
Electronic switching devices of the type under consideration are often so-called proximity switches, which therefore operate without contact. Such switching devices can indicate whether an influencing element to which the corresponding proximity switch is sensitive has approached the proximity switch closely enough. If an influencing element has approached the proximity switch closely enough, a sensor element, via an evaluation circuit, reverses a switch, which is provided on an output side. In the case of a switching device having a make contact, the electronic switch, which is initially nonconductive, becomes conductive. In the case of a switching device having a break contact, the electronic switch, which is initially conductive, is configured for blocking. With such switching devices, whether a physical quantity of an influencing medium to which the switching device is sensitive has reached a corresponding value also can be indicated. Such electronic switching devices can also include an internal operating voltage supply circuit for making available the internally required operating voltage (i.e., internal operating voltage).
Often electronic switching devices are configured as inductive proximity switches. In this case, the sensor element is generally a tuned circuit, which includes a tuned circuit inductance, a tuned circuit capacitance, and which can be influenced by a metallic influencing element. Influencing by the metallic influencing element, then can lead to attenuation and/or to a change of the resonant frequency of the tuned circuit. Often, the damping of the tuned circuit is evaluated; but a change of the resonant frequency of the tuned circuit can also be evaluated.
The machine guideline of the European Union for certain applications of electronic switching devices requires protective measures. Such protective measures, among others, require adherence to certain standards. One such standard is EN 954-1, which was adopted after almost eight years of work, and is entitled “Safety of machinery—Safety-relevant parts of controls, Part 1: General Configuration Principles.” The heart of EN 954-1 is the establishment of five categories for safety-relevant parts of machine controls and protective devices. These categories can be used independently of the application and the control technology used, and can simply be oriented toward the existing risk, which proceeds from the machine. In categories B and 1, the resistivity to faults is achieved largely by use of suitable components to address failure. However, when a fault occurs, the safety function can be ineffective. Category 1, as compared to category B, has a higher resistivity to faults by using special components, which are proven in safety engineering. In categories 2, 3 and 4, improved efficiency with respect to the stipulated safety function is achieved largely by structural measures. In category 2, the execution of the safety function is checked at regular intervals (generally automatically by technical means). Between the test phases, however, when a fault occurs, the safety function can fail. By suitable selection of the test intervals when using category 2, a suitable risk reduction can be achieved. In categories 3 and 4, the occurrence of an individual fault cannot lead to loss of the safety function. In category 4, and whenever feasible in an appropriate manner in category 3, these faults are automatically detected. In category 4, moreover, there is resistivity to the accumulation of unnoticed faults.
Regarding what has been stated above with respect to EN 954-1, reference is made to the BIA Report 6/97 “Categories for safety-relevant controls according to EN 954-1,” published by the General Federation of Professional Associations (ISBN: 3-88383-445-9; ISSN: 0173-0387). There also is EN 60947-5-3 for sensors and for switching devices. This standard differentiates between safety levels PDF-D, PDF-T, PDF-S, and the highest safety level PDF-M.
In a new standard, EN 61508, there are SIL categories, specifically SIL 1 to SIL 4 (SIL=Safety Integrity Level). Category SIL 3 corresponds roughly to category 4 of EN 954-1. In order to reach these safety levels, fault management measures must be implemented. This is achieved by employing an appropriate structure. Category 4 of EN 954-1, which corresponds essentially to SIL 3, can be implemented essentially only with a redundant/diversity structure. Such structure has always been implemented in the prior art only with two sensor elements, often called “elementary sensors,” which is a complex and consequently a costly manner of implementation.